There is conventionally a problem of an increase in brightness in a color CRT particularly used in a high definition display unit such as a computer display and a display unit of a large screen.
A method for increasing a voltage applied to an anode of the CRT, a method for increasing an electronic beam current from a cathode of an electron gun, a method for improving light emitting efficiency of a phosphor itself on a phosphor screen, etc. are known in the prior art for increasing the brightness of the CRT.
However, in the method for increasing the voltage applied to the anode of the CRT, power consumption is increased and there tends to be a critical defect of discharge in the CRT. Further, deflecting efficiency of a deflecting yoke (DY) is reduced and a problem of heat generation of the deflecting yoke is particularly caused in the high definition CRT.
In the method for increasing the electronic beam current from the cathode of the electron gun, a focusing function is generally deteriorated and an emission life of the cathode is shortened, and the life of the electron gun is also shortened. Further, in the high definition CRT, an amplifying circuit of the electronic beam current is formed in a high frequency region and it becomes difficult to increase the electronic beam current itself.
Further, the color CRT basically has three kinds of phosphor stripes (or dots) of red (R), green (G) and blue (B) and light is emitted from the phosphors by hitting the beam from the electron gun against these stripes. If the electronic beam is of high densities as it hits against the phosphor screen in a light emitting region of a visible ray actually used, brightness is correspondingly increased. Namely, it can be said that brightness of a specific phosphor is proportional to the electronic beam current.
However, light emitting efficiencies (bright degrees with respect to the same beam electric current, i.e., brightnesses) of the red, green and blue phosphors are different from each other. The light emitting efficiency of the red phosphor is generally worse than blue and green phosphors with respect to phosphors developed and used at present. Therefore, it is necessary to increase the electronic beam current to increase the red brightness. However, there is also a restriction of the beam electronic current determined from the above-mentioned circuit or the life of the electron gun so that there is a limit in the allowed beam electric current. Accordingly, beam electric currents with respect to green and blue are set such that red, green and blue lights are balanced under a condition in which the red beam electric current is increased as much as possible. Namely, the beam electric currents corresponding to the green and blue phosphors are set to be lower than their allowed maximum electric currents. Thus, the entire brightness of the CRT is actually determined by the red light emitting efficiency.
Further, the method for improving the light emitting efficiency of the phosphor itself on the phosphor screen has been continuously researched conventionally, but has recently attained a maximum state.
It is also considered that brightness can be increased by coating a phosphor layer with a thick coating and relatively increasing a phosphor amount. However, the phosphor stripes on the phosphor screen have recently been thinned to achieve high definition of the CRT so that the phosphor layer is thinned and the phosphor amount is reduced, thereby reducing brightness. FIG. 3B shows a typical phosphor layer of a computer display of 20 inches in size. A size from a phosphor R to the next phosphor R is set to about 0.3 mm. A width wp of each of phosphor stripes of red (R), green (G) and blue (B) seen through a panel glass and a width wc of a carbon stripe are approximately set to about 0.05 mm (50 .mu.m). There is a constant limit in thickness of the phosphor layer when the phosphors are coated along this very narrow width of 50 .mu.m.
It is considered, in order to to improve brightness that a ratio of the carbon stripe width w.sub.c and the phosphor stripe width w.sub.p on the phosphor screen can be changed and the carbon stripe width w.sub.c can be thinned and the phosphor stripe width w.sub.p can be correspondingly increased. However, in this case, a color shift results from caused by a slight mislanding (a shift in position between an electronic beam and each of the phosphor stripes) so that an image quality is reduced. In particular, the phosphor stripe width w.sub.p and the carbon stripe width w.sub.c are thin in the high definition CRT so that a marginal degree of error with respect to the mislanding is slight and it is difficult to increase this phosphor stripe width as a trial.
Therefore, it is considered as a trial that the same phosphors are used and brightness of the CRT is improved by increasing transmittance of each of a panel glass 3 and a safety glass (SP safety glass ) 1 of the CRT arranged between the phosphors and man's eyes shown in FIG. 3A. However, in this case, a problem of a reduction in contrast by the second power of transmittance is caused so that it is difficult to increase this brightness. Accordingly, general transmittance of the safety glass 1 and the panel glass 3 is generally reduced to about 40 to 80% (these contents will be explained later in detail by using FIG. 3C).
An object of the present invention is to provide a CRT for improving brightness without changing the conventional electron gun, the light emitting layer, etc. in the above-mentioned situation.